Synthesis and Characterization of Positively Charged tris-Imidazolium Calix[6]arene Hosts for Anion Recognition
Résumé
Abstract The syntheses of new tripodal ligands with effective C 3 symmetry and anion recognition properties, are reported herein. The ligands have a 1,3,5‐ tris ‐methoxy‐2,4,6‐ tris ‐(α,ω‐bromoalkoxy)calix‐[6]arene as a backbone, and were prepared from 1,3,5‐ tris ‐methoxy‐ p ‐ tert ‐butyl‐calix[6]arene by alkylation with a series of α,ω‐dibromo‐alkanes [Br(CH 2 ) n Br] of different chain lengths (n=2, 4 or 6). Further substitution of the bromo groups, via the use of 1‐methyl‐, 1‐mesityl‐or 2,6‐diisopropylphenyl‐1 H ‐imidazole, led to alkylation by imidazole units and afforded tripodal and tricationic 1,3,5‐ tris ‐methoxy‐2,4,6‐ tris ‐imidazolium‐ p ‐ tert ‐butyl‐calix[6]arenes. These scaffolds provide a framework for H‐bonded and electrostatic interactions with various anions and their anion binding properties were investigated by 1 H NMR and UV‐visible spectroscopy. A sizeable selectivity for HSO 4 – (present as the n ‐Bu 4 N + salt) was demonstrated for most of the tripodal ligands considered in this work. Other anions such BF 4 – , ClO 4 – , PF 6 – , NO 3 – , Cl – exhibited weak to medium associations with the investigated ligands. A single crystal X‐ray study, unfortunately of poor quality, corroborated the geometry of the calix[6]arenes that was previously established by 1 H NMR spectroscopy. The binding‐site geometry was more definitively probed by DFT calculations. Introduction